Photomechanical process



Siler Apn'l 13, 1943. J, A C, YULE PHQTOMECHANIGAL PROCESS Filed Nov. 18, `1942!.

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PRCE'SSING INCL UD/NG WA 5 HOFF POLAR/Z ING IHN AL'. YULE FILTER INVENTOR m M BYV ATTORNEY Patented Apr. 13,1943

rnoToMEcnANIoAL PROCESS John A. .C. Yule, Rochester, N. Y., assignor to Eastman Kodak Company, Rochester, N. Y., a corporation of New Jersey Application November 18, 1941, Serial No. 419,595 in Great Britain .lune 6, 1941 Claims.

The present invention relates to methods of contrast control in photographic reproduction and is most useful in connection with photomechanical reproduction.

In my copending application, Serial No. 330,566, led April 19, 1940, a method of contrast control is described. Primarily, the method consists of placing a screen in Contact with a photosensitive layer on which the image is to be printed and then varying the amount of light transmitted by the screen in proportion to the contrast desired.

The screen elements each include dense and less dense areas and the factor varied is actually the ratio of the amounts of light transmited by these areas, i. e., the apparent contrast of the screen.

in halftone reproductions, the screen is vignetted and varying this ratio (apparent contrast) varies the rate oi' change of dot area with exposure. In continuous tone reproductions the contrast change is marked only when the exposure through the dense areas is so small as to be on the toe and underexposured or even unexposed portions of the characteristic curve of the photosensitive layer used.

Since contrast control nds its greatest use in connection with vignetted screens which constitute contact halftone screens, the present specic invention is not concerned with the other phenomenon and it will not be discussed here. The advantages of contact halftone screens have been known for many years, and one of their disadvantages was the inability to control contrast when using such screens. The process disclosed in the above-mentioned application provides one method of controlling contrast with the screens. The example given in that application is a colored screen whose light transmitting properties are varied by varying the color of the light 'incident thereon. The preferred form of colored screen -is one which is non-diiusing optically as taught by Murray in application, Serial No. 330,567, also led April 19, 1940.

The present invention is an alternative way of realizing this broad invention for contrast control, and the present invention has many unique advantages.

It is an object of the invention to provide a method of controlling contrast with a vignetted halftone screen and to provide a screen for the practicing of this method. A copending application 419,596 which I i'lled concurrently herewith relates specically to the screen.

It is a, specific object of the present invention to provide proper tone rendition when making a black-and-whte copy of any type of origfil corners to the centers.

inal record including multicolored originals.

More specifically it is to give the same tone renpositives from a multicolored original, the color of the printing light is of necessity limited to the color of the separation required; for example, to one of the primary colors and cannot be varied. It is an object of the present invention to provide a method and screen for controlling contrast with a vignetted screen when making color separations. f

Although contrast control can be obtained when using sharp screens in contact with the sensitive surface, the advantages of the present invention are realized in those processes which must have'vignetted screens and the present application is limited to such screens. According to the invention, a vignetted halftone screen is made up in which the elements are graded substantially continuously in light polarizing power from the centers to the corners of each element. Various forms of polarization are known and the present screens may have many of these forms; for example, the corners of the elements may be elliiptically or circularly polarized and the centers of the elements unpolarized, or the type or direction of polarization may vary from the However, the preferred embodiment of the invention employs a screen which varies from unpolarized at the corners to plane polarized at the center of each element. .According to the invention, this vignetted polarized halftone screen is placed in Contact with a sensitive surface which is in printing relation to the continuous tone record to :be reproduced which is either a monochrome or a multicolored record. The printing relation maybe either for contact printing or projection printing. A polarized lter is placed somewhere between the source vof illumination and the screen, i. e., either lbetween the source and the record or between the record and the screen. The apparent contrast of the screen and, hence, the contrast of the results, depends on the orientation of the .polarizing lter. The higher the apparent contrast of the screen, the lower the resulting contrast in the halftone. According to the invention, the polarizing filter is rotated until the apparent contrast of the screen is inversely proportional to the contrast of the continuous tone record, in order to give uniform vcontrast or the desired contrast in the prints. The ratio of the amount of light trans mitted by the corners of the screen elements to using the present, invention.

' filter I5 is indicated by a scale I9 on the mount that transmitted by the centers is a measure of i the apparent contrast. l The polarizing filter and polarizing screen have no effect on the color of the exposing light and hence the invention is particularly suitable when. making color separation records. Thus accordl ing to the invention, a color separation halftone record is made from a multicoloredoriginal by ltering the exposing light through the proper primary color filter and by having a vignetted polo larizing screen in contact with the photosensitive layer on which the color separation record is to I be made, the contrast of the record being coni trolled by proper control ofA the polarization of the printing light.

If the photosensitive layer is a differentially sensitized monopack for making a black printer in accordance with United States Patent 2,183,524,

Yule, the contrast of the black printer is excellently controlled by th'e present invention. 20

When employing the process any polarization 'of the light from the subject before it strikes the filter would introduce extraneous eiects which may be undesirable.l vThese may be climiF nated by placing between the copy and the filter 5 (i. e., in front of the lens in a projection printer) a strongly birefringent material to depolarlze th-e light. Contrariwise, the depolarizer may be used as the control in which case it. is placed between the filterand the screen which are fixed with 30 their vibration axes mutually. at right angles. With no depolarizing of the light, the apparent contrast of the screen is a maximum; with complete depolarization it is a minimum and is very low, the average gradient or density range being g; less than 0.3. v Different degrees of depolarization may be obtained by introducing different amounts of birefringent material between the illter and screen or by rotating a quarter wave plate in this po- 40 sition.

Colored vignetted screens used in the process taught by Serial Number 330,566, mentioned above, can be made by photographic processes and the present screens can be made by the corre- 15 sponding vectographic processes. Such processes are described in the Journal of the Optical Society of America, vol. 30, June, 1940, and in the British Patent 528.796,'assigned to International Polaroid Corporation. One particularly suitable 50 method of making the present vectograph screens, is described below along with a detailed description of theinvention which will be under'- stood when read in connection with the accom- 'Danying drawing in which: v 55 Fig. 1 illustrates a method of contrast control Fig. 2 villustrates an embodiment of the invention applied to contact printing. v

Figs. 3, 4, 5 and 6 constitute a flow chart illus o trating a method of making a vectographic vignetted halftone screen.

Fig. 7 illustrates a variation of theembodimerit in Fig. l.

Fig. 1 illustrates the making of a color separa- 65 tion negative fromy a multicolored original I0 illuminatedv by white light from illumlnators II. Light from the original passes through a color illter I2 and is focused by an objective I3 onto a photosensitive layer I5 in contact with which 70 is placed a vignetted polarizing screen Il accord-4 ing to theinvention. Between the filter I2 and 'the lens 'n is placed a poiarizmg'mter Is carried by a rotatable z nount Il rotatably secured to the lens mount |654 The orientation of the polarizing 75 I1 against an index 20 on the lens mount i8. In practice the color filter I2 is often placed between the lens elements but is here shown spaced from the lens for clarity. In the screen I4, the elements are continuously graded in light-polarizing properties from the corners to the centers thereof and this is indicated schematically by lines of increasing length. The contrast of the resulting picture on the photosensitive layer I5 depends on the apparent contrast of the screen I and this dependson the orientation of the filter I6. The transmission of the unpolarized corners is constant as the lter I is rotated but that of the centers varies from zero to substantiallyk equal to that of the corners so that the total light transmission is least when the apparent contrast of the 'screen is greatest. Therefore, according to the invention to decrease contrast, the polarizing filter I-6 is l rotated so that the axis of this lter is at right angles to the vibration axis of the screen elements. This makes the screen appear quite contrasty and hence gives low contrast results. If higher contrast is desired, the polarizingiilter I6 is rotated -until the vibration axes of the filter and screen are parallel. Intermediate orientations of the filter give intermediate contrasts. In practice the vectographic screens are so nearly perfect that parallel vibration axes make the apparent screen contrast so low that the resultant `halftone contrast is actually beyond the useful range, even though not quite Infinite.

Fig. 2 illustrates the invention applied to .contact printing, and in this case the record zl'beinsr copied is placed against the screen I4 and held there by a transparent plate 28. The printing exposure is provided by a light source immediately in front of which is placed the polarizing filter I6 in its rotatable mount I'I carried on a support 26. Of course, -in the contact printing arrangement, no lenses are necessary.

Figs. 3, 4, and 5 illustrate the making of a vignetted polarizing halftone screen. Illuminationis provided by a lamp 3l having reflector 32 and a diffusing plate such as an opal glass plate 33 through an aperture 31 in a. mask 36.` Light 4from this source passes through a ruled halftone screen 38 placed at halitone distance from a sensitive layer 39 carried on a support 4I). The exposure is through the support 40 and may be varied directly or by any of the usual `methods,

, such as changing the shape of the aperture 31 or by making a succession of exposures through dlilerent apertures. I'he photosensitive layer 39 is. of the wash-o relief type and is processed in the usual wash-ofi manner to give a relief image 4I mounted on the base 40. The wavy line in this case is intended to represent actual physical appearance of a cross section; and for this reason similar lines are not used in representing degreesof polarization such as element I 4 of Figs. 1 and 2 and element of Fig. 6. A

In accordance with the usual method of making vectographs this relief image is then dyed l intimate contact with a vecto sensitive layer I3 mounted on a support M, and gives a vectograph 4I which constitutes a vignetted polarizing halftone screen. In one form of vectograph production, the yyecto sensitiucY lay, 4I is polarized l g. I

process used for the production of the vignetted screen but of' the processes disclosed in British Patent 528,796, accepted November 7, 1940, the one just described appears to be the most satisfactoryl for the present case. The other standarLmethod employs an "ink (polarizing control yso uton) which produces proportional polarization in an oriented sheet leaving unpolarized the points which receive no solution.

In Fig. 7, the filter i6 and the screen I4 are preferably fixed relativevto one another with their vibration axes'crossed.A A depolarizer 50 suchas a rotatable quarter wave plate is placed between the lter and screen so that the apparent contrast of the screen is controlled by controlling theamount of depolarization. Thus the amount rather than the direction of the polarization is the control factor in this embodiment. Having thus described various embodiments of my invention, I wish'to point out that it is not limited to these arrangements but is of the scope of the appended claims.

What I claim and desire to secure by Letters Patent of the United States is:

1. In processes for the photoreproduction of a continuous tone record, the method of controlling Acontrast lwhich comprises placing in printing relation to the record a photosensitive layer'inA contact with the surface of which, which surface faces lthe record, is a contact halftone screen whose elements are graded substantially continuously in light polarizing powerfrom the centers to the corners thereof, polarizing the light from the continuous tone record by a'polarizing filter inthe path of the light, varying the polarization of the light as incident on the screen until the apparent contrast of the screen is inversely prothe continuous tone record by a polarizing filter in the path of the light, varying the direction of polarization of the light as incident on the screen until the apparent contrast of the screen is inversely proportional to the contrast of the continuous tone record, exposing the photosensitive layer through the screen with light from the continuous tone record, so polarized and so varied and developing the layer.

3. In processes for the photoreproduction of a continuous tone record, the method oi controlling contrast Which comprises placing in printing relation to the record a photosensitive layer in contact with the surface of which, which surface faces the record, is a contact halftone screen whose elements are graded substantially continuously in light polarizing power from the centers to the corners thereof, rotating a polarizing filter in front of the photosensitive layer and screen until the apparent contrast of the screen is inversely proportional to the ,contrast of the continuous tone record, exposing the photosensitive layer by light through the lterA and from the continuous tone record and developing the layer.

4. In processes for the photoreproduction of a continuous tone record, thev method of controlling contrast which comprises placing in printing relation to the record a photosensitive layer in contact with the surface of which, which surface faces the record, is a contact halftone screen whose elements are graded from strongly plane polarizing in the centers to` substantially nonpolarlzing in the corners thereof, rotating a polarizing filter in front of the photosensitive layer and screen until the ratio of the intensity of the light through said corners to that through said centers is proportional to the contrast of the continuous tone record, exposing the photosensitive layer by light through the filter and from the continuous tone record and developing .the layer.

multicolored continuous tone record, the process portional to the contrast of the continuous tone record, exposing the photosensitive layer through the screen with light from the continuous tone record, so polarized and so varied and developing the layer.y f

2,-In processes for the photoreproduction of a continuous tone record, the method of controlling contrast which comprises placing in printing relation to the record a photosensitive layer in contact with the surface of which, which surface faces the record, is a contact halftonescreen whose elements are graded substantially continuously in light polarizing power from the centers to the corners thereof, polarizing the light from of making a primary color separation halftone record of predetermined contrast which comprises placing in printing relation to the record a photosensitive layer in contact with the surface of which, which surface faces the record,

is a contact halftone screen whose elements are graded substantially continuously in light'polarizing power from the centers to the' corners thereof, placing a color lter oi' said primary color and a polarizing filter in front of the screen, rotating the polarizing filter to adjust the light transmission of the combination of screen and polarizing filter in accordance with said predetermined contrast, exposing the photosensitive layer by light through both filters, and from the continuous tone 'record and developing the layer.

JOHN A. C. Y'ULE. 

